In recent years, in the field of treating or curing an acute myocardiac infarction or a heart stroke, a method has been employed in which a stent is indwelled in a lesion in a coronary artery (stenosed portion). A similar method may be used to treat or cure a stenosed portion formed in other living lumens, such as other blood vessels, a bile duct, a trachea, an esophagus, a urethra, etc. A stent delivery system used to indwell a stent typically includes a long shaft portion and a balloon which is expandable in a radial direction provided in a distal end side of the shaft portion. A cylindrical stent formed of a metal wire is positioned (mounted) on the outer circumference of the contracted balloon. After the balloon reaches an intended location in the body via a narrow living lumen, the balloon is then expanded to plastically deform and expand the stent, thereby distending the stenosed portion. The balloon is then contracted, though the stent stays expanded, thereby keeping the stenosed portion in a distended state.
For the reason mentioned above, the balloon on which a stent is mounted should be kept contracted and thin until the balloon reaches the intended location. Such method in which a balloon is folded to encircle an outer circumference of a shaft portion in the circumferential direction is known and an example is described in Japanese Patent Application Laid-Open No. 2008-86463.
However, when a balloon is folded to encircle the outer circumference of the shaft portion, portions of the balloon are stacked in layers, and the number of layers of each stack varies from portion to portion, which causes a variation in a thickness of the stacked of layers. Therefore, the outer diameter in a section perpendicular to the axis of the stent mounted on the balloon is rather large, which may make an insertion into a narrow living lumen difficult.